1. Field of the Invention
The present invention relates to a plasma process method including a plasma CVD (Chemical Vapor Deposition) process capable of well forming crystalline or non-monocrystalline, functional deposit films useful to photosensitive devices for electrophotography, line sensors for capture of image, image pickup devices, photovoltaic devices, etc. as semiconductor devices, a sputtering process capable of suitably forming deposit films of insulating films, metal wires, etc. as semiconductor devices or optic elements, or etching of a body to be processed, or the like and also to an apparatus that can carry out the mentioned plasma process. More particularly, the invention concerns the plasma process method and the plasma process apparatus for processing a substrate, particularly using a plasma as an excitation source and, especially, the invention concerns the plasma process method and the plasma process apparatus that can suitably use high frequencies of 450 MHz and less.
2. Related Background Art
Proposed in the technology of element members used for electrophotographic, photosensitive member are a variety of materials including selenium, cadmium sulfide, zinc oxide, phthalocyanine, amorphous silicon (hereinafter referred to as a-Si), and so on.
Among others, non-monocrystalline deposit films containing silicon as a main ingredient typified by a-Si, for example, amorphous deposit films of a-Si or the like compensated by hydrogen and/or halogen (for example, fluorine, chlorine, or the like), are proposed as high-performance, high-durability, and nonpolluting photosensitive members, some of which are used practically.
U.S. Pat. No. 4,265,991 discloses the technology of the electrophotographic, photosensitive member a photoconductive layer of which is made mainly of a-Si.
The a-Si photosensitive members have high surface hardness, demonstrate high sensitivity also to long-wavelength light such as semiconductor lasers (770 nm to 800 nm), and exhibit little deterioration after repetitive use, and they are widely used, particularly, as photosensitive members for electrophotography in high-speed copiers, LBPs (laser beam printers), and the like.
As methods for forming such deposit films there are many conventional methods known, including the sputtering process, the method for decomposing raw-material gas by heat (the thermal CVD process), the method for decomposing the raw-material gas by light (the photo-CVD process), the method for decomposing the raw-material gas by plasma (the plasma CVD process), and so on.
Among them, the plasma CVD process, which is a method for decomposing the raw-material gas by glow discharge or the like induced by direct current or high-frequency (RF (Radio Frequency) or VHF (Very High Frequency)) microwave and forming a deposit film of thin film on a substrate of glass, quartz, heat-resistive, synthetic resin film, stainless steel, aluminum, or the like, is used for formation of the a-Si deposit films or the like used not only for electrophotography but also for many semiconductor devices and is under very quick development into practical use. A variety of proposals were also made on apparatus for the process.
Further, demands are becoming stronger for improvements in quality of film and in throughput and many ideas are under research.
Especially, the plasma process using high-frequency power is used because of its various advantages including high stability of discharge, applicability to formation of insulating material such as an oxide film or a nitride film, and so on.
Meanwhile, the copiers of nowadays are required to have high performance and high functionality.
One of such pursuit of high performance is an increase in copy speed. The higher the speed, the shorter the time that can be used for electrification, resulting in the tendency to lower the charge potential. Since the time for exposure must be also decreased similarly, higher sensitivity is also required at the same quantity of light.
One of pursuit of high functionality is multicolor copy. In this case, the distance tends to increase between a charging device and a developing device because of a need for mounting of plural developing devices, so that the surface potential is attenuated during that period, thus tending to lower the charge potential at the position of developing device.
As described above, further improvements in the total characteristics including electrifiability are desired for the electrophotographic, photosensitive member using the a-Si film.
Conventionally, deposition rates for obtaining the a-Si film satisfying the performance of photosensitive member for electrophotography were, for example, those of approximately 0.5 to 6 xcexcm per hour, and higher deposition rates than those might result in failing to achieve sufficient characteristics for the photosensitive member.
In general, in the case of the a-Si film being utilized as a photosensitive member for electrophotography, at least the film thickness of 20 to 30 xcexcm is necessary for achieving sufficient electrifiability because of the magnitude of its dielectric constant. This was a cause to raise the fabrication cost, because a long time was necessary for fabrication of photosensitive member for electrophotography. Therefore, there were strong demands for the technology to reduce the fabrication time without degrading the characteristics of the photosensitive member.
One method to meet the recent demand for an improvement in electrifiability and the recent demand for a reduction in deposition time is a report of the plasma CVD process using a high-frequency power supply of 50 MHz or more with a diode parallel plate plasma enhanced CVD system (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3 (1987) pp. 267-273), which indicates the possibility of increasing the deposition rate without degrading the performance of deposit film by using the higher discharge frequency than 13.56 MHz used conventionally and which is drawing attention.
Further, JP-A-3-64466 presents a report on application thereof to the electrophotographic, photosensitive member and research thereof is widely conducted in recent years.
The present inventors investigated the plasma CVD process using the high-frequency power at the high frequency in the so-called VHF band of 50 MHz or more and results thereof demonstrated that an increase in the deposition rate was achieved actually and that an improvement was also observed in the performance of deposit film.
This method, however, has the possibility of raising the following problems at the same time.
Specifically, the normal plasma CVD apparatus is provided with a matching circuit for impedance matching between the high-frequency power supply and the load, but, because impedances are distributed in the discharge electrode in the VHF band, the plasma becomes nonuniform in some cases.
Therefore, the most deposition systems are arranged so that not only the matching circuit but also the discharge electrode itself are optimized so as to keep the plasma discharge uniform and so as to suppress the film thickness distribution of deposit film. Especially in the VHF band, in order to prevent the distribution of impedance in the electrode to which the high-frequency power is applied, the area of the electrode is decreased in many cases.
With the thus optimized apparatus satisfactory results were obtained in film formation of device.
In any device deposition systems by vapor deposition, as well as by the plasma CVD process, unless after fabrication of device a deposit film on a deposition chamber etc. other than the substrate is removed by any method, it will generate particles due to exfoliation or the like in next fabrication, which lowers the non-defective rate of products.
Especially, in the case of devices requiring a thick deposit film like the electrophotographic, photosensitive member, an amount of deposits depositing on the other portions than the substrate, per process is large and cleaning is necessitated every time or at very short cycles unless the configuration of the deposition chamber is designed with special care.
A usually employed method is a method of dry etching for causing plasma discharge in an ambience of fluorine-based gas and gasifying the deposit silicon film thereby so as to remove it, as disclosed in the bulletin of JP-A-59-142839.
However, a problem herein is that the plasma CVD process has such a property that the matching conditions differ depending upon the type of gas used.
For example, in the case of film formation of the a-Si film, a silicon-based gas is used upon formation of film and a fluorine-based gas is used in cleaning as described previously.
However, because the properties of these gases are greatly different, when the plasma discharge of the fluorine gas is excited in the film forming apparatus optimized for uniforming the plasma discharge of the silicon-based gas, uniformity of plasma is destroyed in many cases. This raised a problem that a very long time was necessary for completely removing the deposit film because of localization of plasma when the technique of dry etching described above was applied to cleaning of the deposit film depositing on the inside of the film-forming chamber.
Even in the case of nearly perfect cleaning being achieved, a film also remained at some positions where the plasma was extremely weak, and it became a cause of generating dust upon next film formation, thus degrading image defects in some cases.
Further, the deposit film remaining every process is stacked one over another with repetition of process. After repetition of a certain number of processes, the deposit films must be cleaned off separately by man hand. This additional cleaning raised problems of a decrease in availability of apparatus and a need for labor.
Further, in the normal dry etching process O2 is added in order to raise the etch rate.
For example, in the case of CF4 being used as the fluorine-based gas, dry etching is conducted with addition of O2 gas at a flow rate equal to approximately 10 to 30% of CF4, which can increase an amount of generation of F radicals and which can increase the etch rate.
This case, however, might have a problem that when a diffusion pump is used as an evacuation device, oxygen contacts boiling DP (Diffusion Pump) oil to cause oxidation of oil, thereby greatly decreasing the life of diffusion pump.
A conceivable method for overcoming it is a method for evacuating the chamber by only a mechanical booster pump and a rotary pump as bypassing the diffusion pump only upon dry etching, but in this case, the pressure in the film-forming chamber increases up to considerably high pressure, 0.1 Torr or more.
When the plasma is generated at the high frequency of 50 MHz or more, the localization of plasma becomes worse where the pressure in the discharge space is 0.1 Torr or more. This raises a problem that the etching unevenness described previously becomes further worse in some cases.
It is, therefore, an object of the present invention to provide a plasma process method and a plasma process apparatus, solving the above problems, that are optimum for forming a desired deposit film, for example, for fabricating an electrophotographic, photosensitive member capable of achieving a high charge potential upon electrification and thus obtaining a high-density image.
It is another object of the present invention to provide a plasma process method and a plasma process apparatus that are optimum for fabricating a deposit film with excellent reproducibility, especially a photosensitive member for electrophotography, in short fabrication time and at low cost.
It is a further object of the present invention to provide a plasma process method and a plasma process apparatus that can decrease the cleaning time remarkably in cleaning the deposit film depositing inside the deposition chamber by the dry etching technique.
It is a still further object of the present invention to provide a plasma process method and a plasma process apparatus that can form a deposit film with less defects, in uniform film thickness, and with uniform characteristics, for example, a deposit film used in a photosensitive member for electrophotography, capable of obtaining clear images with less image defects.
It is a still further object of the present invention to provide a plasma cleaning process method, which comprises introducing a raw-material gas into a deposition chamber while evacuating the inside of the deposition chamber capable of being kept airtight in a vacuum, decomposing the raw-material gas by high-frequency power in the VHF band, performing film formation of a deposit film on a substrate set in the deposition chamber, and thereafter cleaning the inside of the deposition chamber, wherein said film formation is carried out based on decomposition using the high-frequency power in the VHF band, and wherein said cleaning is removing by etching of an unnecessary deposit film depositing inside the deposition chamber, using a gas comprising fluorine and supplying high-frequency power of a lower frequency than that of the high-frequency power upon the film formation.
It is a still further object of the present invention to provide a plasma process apparatus comprising a deposition chamber having an inlet port for introducing a raw-material gas or an etching gas and an exhaust port for evacuating the inside to a vacuum, and a high-frequency power supply for supplying high-frequency power for generating a plasma in the deposition chamber, wherein the high-frequency power supply is arranged to be capable of supplying high-frequency power in the VHF band and high-frequency power in a lower frequency band than the VHF band.
The present invention can achieve the objects of the present invention described above, which is based on the following knowledge of the present inventors.
Namely, the present inventors have conducted extensive and intensive research on the above problems in use of the high-frequency power supply of 50 MHz or more.
In general, when plasma discharge takes place at a high frequency in the VHF band of 50 MHz or more, the electrode to which the power is applied is made in a smaller electrode area than those used at discharge frequencies in the normal RF band, in order to suppress the impedance distribution in the electrode.
With such a smaller-area cathode electrode the balance of optimum values is lost with a great change in the type of gas subjected to discharge, thereby producing a distribution of plasma.
Further, in the case of a deposit film forming apparatus using the diffusion pump in its exhaust system, use of oxygen-based gas is not allowed and in the case of a deposit film forming apparatus using a turbo molecular pump, a negative effect could appear due to dust of SiO2 generated upon etching. For this reason, evacuation is often carried out using the mechanical booster pump and the rotary pump while bypassing the diffusion pump or the turbo molecular pump upon dry etching, so that the vacuum in the discharge space must become 0.1 Torr to several Torr.
Therefore, further localization of plasma occurs in discharge in the VHF region, likely to localize the plasma, and it is thus difficult to effect uniform dry etching throughout the entire region in the apparatus.
The present inventors have conducted extensive and intensive research on means for dry etching with an apparatus having a far smaller cathode area than that of the normal RF plasma CVD apparatus, which is capable of producing a uniform plasma under the internal pressure in the discharge space being in a high pressure range of 0.1 Torr to several Torr, thereby effecting uniform dry etching throughout the deposition chamber and at relatively high etch rates. After checking various discharge frequencies and various types of etchant gases, the inventors found out that as to the dry etching, the plasma spread all over the deposition chamber even under the above discharge conditions, thereby uniformly etching the deposit film where the high-frequency power was in the RF band of 20 MHz or less but not in the VHF band and where the etchant gas was a gas containing fluorine, especially, at least one of CF4, CFmHn (m+n=4), and ClF3, combined, if necessary, with O2, Ar, N2, or H2.
From the above knowledge, the present inventors have come to find out that for fabricating relatively thick devices like the electrophotographic, photosensitive member in short time and at low cost, upon film formation with a gas compound containing at least silicon, the film was able to be formed uniformly at higher deposition rates than before and the quality of deposit film was able to be also improved, by applying the power at a higher frequency of 50 MHz or more to the optimized electrode of small area.
Especially, the inventors have found that upon cleaning of the deposition chamber after completion of one batch of film formation, the entire space in the deposition chamber (in the film-forming furnace) was able to be dry-etched uniformly whereby cleaning was able to be made in the shortest time and without a remaining deposit film, by applying the high-frequency power at a lower frequency of 20 MHz or less and keeping the discharge space under high pressure of 0.1 Torr or more and by causing discharge in a gas ambience containing at least one of CF4, CFmHn (m+n=4), and ClF3 and, if necessary, further containing O2, Ar, N2, or H2.
In the present invention, preferably, the internal pressure in the deposition chamber upon the film formation is between 0.1 mTorr and 100 mTorr and the internal pressure in the deposition chamber upon the cleaning is between 0.1 Torr and 10 Torr.
In the present invention, the frequency of the high-frequency power upon the film formation is preferably between 50 MHz and 450 MHz both inclusive and more preferably between 50 MHz and 105 MHz both inclusive.
In the present invention, the frequency of the high-frequency power upon the cleaning is preferably between 1 MHz and 20 MHz both inclusive and more preferably, 13.56 MHz.